1. Field of the Invention
Generally, the present disclosure relates to the field of fabricating microstructure devices, such as integrated circuits, and, more particularly, to the coordination of substrates during metrology and process sequences.
2. Description of the Related Art
Today's global market forces manufacturers of mass products to offer high quality products at a low price. It is thus important to improve yield and process efficiency to minimize production costs. This holds especially true in the field of semiconductor fabrication, since, here, it is essential to combine cutting edge technology with mass production techniques. It is, therefore, the goal of semiconductor manufacturers to reduce the consumption of raw materials and consumables while at the same time improve process tool utilization. The latter aspect is especially important, since, in modern semiconductor facilities, equipment is required which is extremely cost intensive and represents the dominant part of the total production costs. Consequently, high tool utilization in combination with a high production yield, i.e., with a high ratio of good devices to faulty devices, results in increased profitability.
Integrated circuits are typically manufactured in automated or semi-automated facilities, thereby passing through a large number of process and metrology steps to complete the device. The number and the type of process steps and metrology steps a semiconductor device has to go through depends on the specifics of the semiconductor device to be fabricated. A usual process flow for an integrated circuit may include a plurality of photolithography steps to image a circuit pattern for a specific device layer into a resist layer, which is subsequently patterned to form a resist mask used in further processes for forming device features in the device layer under consideration by, for example, etch, implant, deposition, polish processes and the like. Thus, layer after layer, a plurality of process steps are performed based on a specific lithographic mask set for the various layers of the specified device. For instance, a sophisticated CPU requires several hundred process steps, each of which has to be carried out within specified process margins so as to fulfill the specifications for the device under consideration. Since many of these processes are very critical, a plurality of metrology steps have to be performed to efficiently control the process flow. Typical metrology processes may include the measurement of layer thickness, the determination of dimensions of critical features, such as the gate length of transistors, for instance, in the form of a resist feature, and, after actually forming the gate electrode on the basis of the resist feature, the measurement of dopant profiles, the number, the size and the type of defects, electrical characteristics and the like. As the majority of the process margins are device specific, many of the metrology processes and the actual manufacturing processes are specifically designed for the device under consideration and require specific parameter settings at the adequate metrology and process tools.
In a semiconductor facility, a plurality of different product types are usually manufactured at the same time, such as memory chips of different design and storage capacity, CPUs of different design and operating speed and the like, wherein the number of different product types may even reach one hundred and more in production lines for manufacturing ASICs (application specific ICs). Since each of the different product types may require a specific process flow, it may be necessary to use different mask sets for the lithography, specific settings in the various process tools, such as deposition tools, etch tools, implantation tools, chemical mechanical polishing (CMP) tools, metrology tools and the like. Consequently, a plurality of different tool parameter settings and product types may be encountered simultaneously in a manufacturing environment, thereby also creating a huge amount of measurement data that are typically categorized in accordance with the product types, process flow specifics, process tools, substrate positions in the respective carriers and the like.
Generally, great efforts are made to monitor the process flow in the semiconductor plant with respect to yield affecting processes or process sequences in order to reduce undue processing of defective devices and to identify flaws in process flows and process tools. For example, at many stages of the production process, inspection steps are implemented for monitoring the status of the devices. Moreover, other measurement data may be generated for controlling various processes, in which the measurement data may be used as feedforward and/or feedback data. Desirably, the measurement data would be generated after each process step in order to verify the result of the corresponding process and use the respective measurement data for controlling the process to minimize the deviation between the measured output and the target process result. However, such a control regime is typically not compatible with economic constraints in many process sequences, since a dramatic reduction in throughput would result, and also significant additional resources in terms of metrology tools would be required. Therefore, advanced process control regimes have been developed which provide a predictive behavior for the processes under consideration with a moderate amount of input measurement data. In this way, a compromise is obtained with respect to delays and efforts generated by the metrology processes, while nevertheless providing efficient process monitoring and controllability. Thus, it is of great importance to generate the corresponding measurement data in an appropriate manner so as to obtain a broad “coverage” of the process status within the highly complex manufacturing environment while on the other hand maintain high yield and throughput.
With reference to FIG. 1, a typical process flow within a portion of a highly complex manufacturing environment, such as a semiconductor facility, will now be described in more detail in order to more clearly demonstrate the problems involved in controlling and monitoring a respective manufacturing sequence.
FIG. 1 schematically illustrates part of a manufacturing environment 100, which may be configured for fabricating complex microstructure devices, such as integrated circuits and the like. In the example shown, a respective process sequence may be illustrated in which substrates or wafers may be processed so as to obtain a respective resist feature, on the basis of which a corresponding underlying material layer may be patterned in order to receive a respective feature, such as a gate electrode of a field effect transistor, which may represent an essential component of highly complex logic circuits, such as CPUs and the like. Typically, in the manufacturing environment 100, respective substrates 101 may be handled and conveyed in respective transport carriers, wherein, typically, a predefined number of substrates 101 may be contained in a corresponding carrier, wherein the group of respective substrates 101 may be referred to as a lot. In semiconductor production, currently a typical lot size may be 25 substrates per carrier. Furthermore, a portion of the environment 100 may comprise a lithography module 110, in which any appropriate process tools and equipment may be provided for forming a corresponding resist layer on the substrates and exposing and developing the corresponding resist layer in order to obtain a respective resist feature, such as a resist feature representing a gate electrode, or any other appropriate structure under consideration. It should be appreciated that the module 110 may comprise a plurality of lithography tools and associated process tools for pre- and post-exposure treatments, wherein each of the various lithography tools may be controlled by an advanced control system 120, which is schematically illustrated as an APC block 120.
The lithography process is a highly complex and important process step and hence the corresponding process result is monitored by a corresponding metrology system 130, which may be configured to determine the respective dimension of the resist feature as obtained after processing by the lithography module 110, also referred to as DICD (developed inspection critical dimension). For example, based on the measurement results of the metrology step 130, it may be decided whether or not respective substrates have to be reworked or whether the corresponding substrates may proceed to the next process step, i.e., to an etch module 140, in which the corresponding resist feature provided by the lithography module 110 may be used for actually patterning the respective material layer, above which are formed the corresponding resist features. The etch module 140 may include a plurality of respective etch chambers, which may be provided in the form of a plurality of individual etch tools and/or in the form of respective multi-chamber etch tools and the like. Since the lithography module 110, in combination with the etch module 140, may determine the finally obtained resolution for forming the critical features, i.e., the consistent capability of creating the respective features under consideration, such as gate electrodes, within a well-defined process margin, a respective metrology module 150 may also be provided after the etch module 140 in order to obtain respective measurement data of the final process output, which is also referred to as final inspection of critical dimensions (FICD).
As previously explained, although it is desirable in view of enhanced process monitoring and process control to obtain respective measurement data on the basis of each substrate and even on the basis of each exposure step for each of the substrates 101, in practice the measurement activity has to be restricted to a reduced number of substrates per lot in order to not unduly increase cycle time within the manufacturing environment 100. Consequently, respective substrates or even measurement sites have to be selected for each lot and for each metrology step. Thus, respective “samples” may be selected and may be used, for instance, in the metrology module 130 for producing respective measurement data indicating the quality of the output of the lithography module 110. Since the respective measurement data may also be used by the controller 120, as well as for further process monitoring, for instance in the form of yield loss estimation, engineering purposes and the like, corresponding measurement data may also be supplied to respective modules 160, 170.
As previously explained, in complex situations, the selection of respective sample substrates may have to be performed to provide a broad coverage with respect to any process situation, while additionally reduce any delay in the overall process sequence in order to maintain a high overall throughput. For this purpose, typically an advanced sampling system 180 may be provided, which may have implemented therein a plurality of “sampling rules or ruleset”, i.e., respective rules for selecting an appropriate set of the substrates 101 per lot on the basis of the process-specific situation that is to be covered by the subsequent metrology module, such as the module 130 or 150. For this purpose, the sampling system 180 may receive appropriate process information for estimating the “context” of the substrate to be measured, for instance of the lithography module 110 so as to obtain the desired information on the basis of an appropriately selected number of sample substrates. For example, if a given number of sample substrates is determined for the metrology module 130, for instance on the basis of process flow specific constraints, the sampling system 180 selects, on the basis of the additional context information, one or more appropriate substrates of each lot in order to provide appropriate measurement data over a plurality of lots so that the corresponding modules 120, 160 and 170 may produce appropriate outputs for controlling the modules 110, 140 for determining yield loss generating mechanisms and the like. For instance, if three substrates per lot is the corresponding sample number for the metrology module 130, the sampling system 180 may select respective three substrates of each lot such that substrates may be measured, after processing a plurality of lots, in a manner that each lithography tool, respective associated pre- and post-treatments and the like may be measured. In this manner, respective information for efficient control of the respective process tools in the module 110 by the controller 120 may be generated. For example, in advanced APC strategies, the corresponding measurement data may be categorized, for instance with respect to specific tool combinations in the respective process module, for instance, a combination of a specific lithography tool in combination with a post-exposure bake tool, so that the sampling system 180 may select the respective sample substrates for producing measurement results with respect to each item of the corresponding categories after a reasonable number of processed lots.
A similar situation may be encountered after the etch module 140, wherein the sampling system 180 may use a different sampling regime based on a different ruleset in order to account for the specific process situation prior to the metrology module 150. For example, in view of enhanced process control, it may be necessary to measure the same samples as in the module 130, for instance so as to detect or eliminate respective systematic drifts which may be important aspects for the respective APC regimes for the modules 110 and 140. Hence, the sampling system 180 may select a respective substrate previously measured prior to forwarding the substrate to the module 150, wherein additional constraints may be imposed on the selection of additional sample substrates, since the previously chosen sample substrates may not necessarily comply with the requirements dictated by the etch module 140, in particular, when a corresponding randomization step has been performed on the basis of a respective randomization system 190, which may be provided in order to obtain a certain decoupling between highly critical process steps. The randomization system 190 may comprise a randomizing unit 195 for generating random wafer positions for a specific lot in combination with a corresponding wafer positioning unit 196, which may act on the corresponding substrates so as to provide a respective wafer sorting within the respective transport carrier based on the random positions created by the unit 195. In this way, the possible “amplification” of systematic drifts and deviations in the modules 110 and 140 may be reduced.
During a typical operational situation in the environment 100, respective lots of substrates 101 may be continuously entered into the lithography module 110, which may provide respective processed substrates for which the sampling system 180 may select respective candidates for being subjected to measurement within the module 130. The sampling system 180 may select, in a highly dynamic manner, the respective sample substrates so as to provide a desired coverage, even though only a small number of sample substrates per lot may be used. Thereafter, the corresponding wafer resorting may be performed on the basis of a randomization process by the system 190 and, thereafter, the respective randomized lot may enter the etch module 140. In the etch module 140, the respective substrates 101 may be processed by a plurality of etch chambers on the basis of their random position in the respective transport carriers, and this context information may be provided to the sampling system 180 in order to obtain, in a correspondingly adapted manner, an appropriate subset of sample substrates that have to be subjected to a measurement process in the metrology module 150. Typically, the sampling ruleset used in selecting sample substrates prior to the module 130 may not be uncorrelated with the rules used for selecting the sample substrates prior to the metrology module 150 since, as previously explained, for example, the same sample substrates may have to be measured in both modules 130 and 150, for instance with respect to eliminating systematic deviations. Consequently, by selecting the same sample substrates, usually respective process situations in the etch module 140 may not be appropriately covered and, thus, corresponding information may not be available for the modules 120, 160 and 170.
For instance, as to the module 130, the substrates 1, 12 and 20 may be selected as appropriate samples, for instance with respect to two lithography tools that may be used in the lithography module 110, wherein the respective substrates 1, 12 and 20 may be processed by different post-exposure tools. In a subsequent lot, the sampling system 180 may also choose three respective substrates having been processed by the different lithography tools and by different post-exposure tools, for instance when two lithography tools and six post-exposure tools may be used in the module 110. Consequently, in this case, a coverage of the six process tracks defined by the involved process tools would be obtained after two processed lots. It should be appreciated that other criteria may also be applied, for instance the position of the respective substrates in the transport carrier and the like. Since the same substrates 1, 12 and 20 have to be measured for the specific lot under consideration after the etch module 140, these substrates may not, however, provide the desired coverage of the process situation in the etch module 140. For instance, two or three of these substrates may have been processed by the same etch chamber, thereby providing a highly unbalanced amount of measurement data, while possibly not covering the respective operational behavior of other etch chambers in the module 140. In this case, the created measurement data may be less efficient, since certain process situations may be “over determined,” while effective measurement data for other situations may be available after a significant delay, after having processed a respective number of lots until, finally, the corresponding process situation may be covered. Thus, in some situations, the sampling system 180 may additionally select further sample substrates, such as the substrates 5 and 9, in order to provide an increased coverage of the process situation in the etch module 140. In this case, significant additional measurement resources are required and may therefore result in a significantly reduced overall throughput in the manufacturing environment 100.
It should be appreciated that the above process sequence is an illustrative example for many other respective process situations, in which coupled metrology steps have to be performed on the basis of respective sampling criteria and, in situations as described above, the respective measurement data may not be efficiently generated, thereby a loss of information may occur, or additional measurement activities have to be performed, resulting in a reduced overall throughput.
The present disclosure is directed to various methods and systems that may avoid, or at least reduce, the effects of one or more of the problems identified above.